The digestion of titaniferous materials with sulfuric acid is one of the two processes to produce titanium pigment. The raw materials used in the sulfate process are essentially ilmenite ores and titaniferous slags produced by electric furnace smelting of ilmenite ores. Ilmenite ores are composed principally of iorn oxides (Fe++, Fe+++) and titanium oxide. Pure ilmenite can be represented by the formula FeTiO.sub.3. Of course, natural ores do not correspond exactly to this formula. The major sources of ilmenite ores are Allard Lake Ore, Australian beach sands and Richards Bay beach sands.
In the sulfuric acid digestion of ilmenite ores, the ilmenite (FeTiO.sub.3) is reacted with sulfuric acid to produce FeSO.sub.4, TiOSO.sub.4 and Ti(SO.sub.4).sub.2. Most of the FeSO.sub.4 is removed as crystals and the liquor is boiled and seeded which precipitates TiO.sub.2 and regenerates H.sub.2 SO.sub.4.
When ilmenite is digested with sulfuric acid, the resulting cake after water leaching yields a liquor containing some ferric sulfate which must be later reduced into ferrous sulfate by reaction with metallic iron. This step is necessary in order to avoid any precipitation of iron oxide and other impurities such as vanadium oxide with the titanium dioxide during the hydrolysis stage.
When titaniferous slag is the raw material for digestion, the sulfate liquor normally contains no ferric sulfate but it contains some reduced titanium. Ti.sup.+3 is present, generally in concentrations of about 3-6 g/l for a titanium concentration of 220-240 g/l expressed as TiO.sub.2. If some ferric ions are present, scrap iron is added to get a reduced solution containing some Ti.sup.+3 as is the case for treating ilmenite. If Ti.sup.+3, expressed as TiO.sub.2 exceeds 6 g/l, the excess Ti.sup.+3 has to be oxidized to avoid losses of titanium as Ti.sup.+3 does not hydrolyse.
Certain ores such as Allard Lake ores (Quebec, Canada) and Richards Bay ore (South Africa) are processed more economically by first smelting the ore with carbon, coal, etc. The process takes place in an electric furnace wherein the ore is liquefied and a substantial part of the iron content thereof is reduced to the molten elementary state. The titaniferous phase floats on this iron and is tapped therefrom into molds, in which it is partly cooled. The products of this process are metallic iron and a slag much richer in TiO.sub.2. The slag produced by the foregoing smelting process when Allard Lake ores are treated is called Sorelslag. This slag and similar slags are treated by the sulfate process described above to recover TiO.sub.2.
Sorelslag upon digestion normally results in the retention of 3-6 g/l of Ti.sup.+3 in the sulfate liquor. Slight variations occur from time to time as in all commercial operations and slight adjustments have to be made sometimes either by reduction with scrap iron or oxidation with chemicals. These chemicals might be nitrates, hydrogen peroxide, etc.
As stated above, the Ti.sup.+3 level of Sorelslag is 3-6 g/l. However, with higher grade ilmenites such as beach sand ilmenite from Richards Bay, a higher TiO.sub.2 slag is obtained in the smelting operation along with a much higher reduced titanium content in the slag. Such a slag will digest properly but will consistently yield a sulfate solution after digestion with a higher Ti.sup.+3 content than the desired (6 g/l). Oxidation with an oxidizing chemical such as sodium nitrate will be required.
Another method to oxidize the excess Ti.sup.+3 content is to digest a mixture of slag and ilmenite (containing ferric oxide) or ferric oxide (U.S. Pat. No. 2,953,434). Such mixtures yield ferric sulfate by reacting with sulfuric acid. During the dissolution of the cake, Fe.sup.+3 ions react with Ti.sup.+3 ions to yield Fe.sup.+2 and Ti.sup.+4. It is possible to digest ilmenite and slag separately and later mix the resulting sulfate liquors in the proper proportions. The disadvantage of this method, however, is that it produces more iron sulfate thus aggravating pollution problems.
U.S. Pat. No. 2,990,250 involves the selective preoxidation of the slag, which method requires treating the finely ground slag at carefully controlled temperatures to avoid the conversion of the slag into an insoluble material (U.S. Pat. No. 2,715,501).
U.S. Pat. Nos. 2,589,909 and 2,589,910 suggest the oxidation of Ti.sup.+3 by aeration during the baking of the sulfate cake. This requires blowing hot air in order to avoid a rapid cooling of the cake.
U.S. Pat. No. 2,850,357 uses a carbonaceous material (coal, coke, carbonized carboxyhydrate) as a catalyst promoting the oxidizing properties of sulfuric acid at high temperature. The disadvantage is that the method needs a very finely divided substance uniformly mixed with the slag which is relatively difficult to realize in commercial operation.